In a communication system, data is transmitted as a plurality of data symbols in data or radio frames. The signals carrying the data may be transmitted with variable data symbol transmission rates (data speeds) and in some arrangements the transmission rate can be different in diffecent frames of the transmission. For example, in a cellular CDMA (code division multiple access) system data is encoded for transmission by processing data symbols to be transmitted by a spreading code for each transmission channel. The effect of a spreading code is to spread the frequency band of a transmission to a chip rate which la larger than the actual data or information symbol rate. This results in more symbols being transmitted than the actual number of information symbols. For example, if the used spreading factor is 8, 8 symbols (referred to as “chips”) are transmitted for every information symbol. It has been proposed that the number of “chips” per data symbol be defined by a spreading factor. The length of the spreading code may however be longer or shorter than the information symbol time. The spreading factor is sometimes expressed by a definition the following ratios of chip rate: data symbol rate or data symbol duration: chip duration where the data symbol duration equals 1: data symbol rate and the chip duration equals 1: chip rate. The terra spreading factor will be used in this document although other terms such as spreading ratio or processing gain may sometimes also be used in this context.
It is possible to set up a variable rate (multi rate) connection where the data symbol rate of the information symbols or bits, and thus the spreading code used in the spreading modulation of the symbols, may vary from frame to frame. The data rates used in such a connection are not arbitrary, but for each frame duration one of the plurality of predefined data rates is used.
Information on the spreading factor used will not necessarily be known by a receiver. The receiver can carry out a process in which the spreading factor is determined. The spreading factor needs to be known in order to correctly process the received data.
In CDMA systems, multi access interference (MAI) is usually present. In a CDMA system, a relatively large number of users will use the same frequency band. The users are distinguished by different scrambling or spreading codes. The terms “scrambling code” and “spreading code” are used the same sense as in the 3GPP specification (see 3G TS 25.213, V3.2.0, 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Spreading and modulation (FDD), Release 1999, page 7). However, interference to a given user will be caused by the other users who use the same frequency. To suppress interference caused by other users, multi user detection (MUD) or interference cancellation (IC) receivers have been proposed. These receivers jointly detect the data symbols of users. The multi user detection or interference cancellation is performed at the base band frequency. This joint detection is currently assumed only to be possible if the spreading factors for all of the users are known. Accordingly, it has been assumed that if one of the users has an unknown spreading factor it is not possible to use multi user detection or interference cancellation techniques.